It is known to heat a conductive liquid by circulating an electric current therein through a pair of electrodes, the conductive liquid being the resistive element which is electrically heated. This is called ohmic or resistive heating and has been applied to the sterilisation of foodstuffs such as fruit juices. With this technology heating is more uniform and can be completed in a very short time, but problems may arise.
For instance, if current density (electric current divided by area of electrode) is too high arcing may occur, leading to the heating of the electrode and the consequent pollution of the foodstuffs with particles from the electrode. Arcing is the occurrence of an electric arc, i.e. an electrical breakdown of a gas resulting from a current flowing through normally non-conductive media, such as air.
U.S. Pat. No. 5,583,960 acknowledges that “many of the difficulties encountered heretofore in electroheating have been caused by phenomena occurring at and adjacent the electrode surfaces when the electrodes are subjected to relatively high current densities”, and discloses an apparatus that “may include a dielectric structure defining an elongated first conduit having inlet and outlet ends and may also include means defining first and second electrode surfaces disposed adjacent to ends of the first conduit so that a conductive fluid material passing through the first conduit will contact the first and second electrode surfaces ( . . . ) both of the electrode surfaces are disposed outside of the adjacent end of the first conduit and at a substantially uniform distance from the conduit and each of the electrode surfaces has area greater than the mean cross-sectional area of the conduit ( . . . ) each electrode surface is generally in the form of a surface region of a sphere having its centre on the central axis of the adjacent conduit end ( . . . ) the dielectric structure desirably includes a transition section associated with each end of the conduit, the transition section extending from the end of the conduit towards the electrode surface of the electrode associated with such conduit end ( . . . ) this wall structure may be generally in the form of a surface of revolution such as a cone, paraboloid or the like having progressively increasing diameter in the direction from the end of the conduit towards the electrode surface ( . . . ) and is connected to the electrode around the periphery of the electrode surface. The electrode may have one or more ports extending through the electrode surface so that a conductive fluid to be heated can be passed through the port of one electrode, through one transition conduit, through the first conduit and through the other transition conduit and the port of the other electrode ( . . . ) the axes of the ports slope in the same direction with respect to the central axis of the conduit, so that the ports are disposed in a generally helical pattern”, in view to reduce the current density on the electrodes' surface.
But found and disclosed here is the development that when heating a particulate liquid (for example orange juice with pulp stuff) with the apparatus of U.S. Pat. No. 5,583,960, both calcined pulp and particles of electrode appear in the heated liquid, and after some time the outer surface of the electrode that is in contact with the liquid is corroded, specially at the periphery. This last detail is particularly worrying because there is a seal between the flat periphery of said surface of the electrode and the transition section of the dielectric structure, and thus the damage to the electrode can also be damaging to the seal.